Electrospray techniques may be used for analysis of sample solutions in applications and systems, such as liquid chromatography (LC) and mass spectrometry (MS). In such techniques, a sample solution (such as the effluent from a LC system) may be atomized by an electrospray device and then analyzed with a mass spectrometer. Such techniques are also often used for analysis of biological specimens or samples.
Electrospray generally refers to methods in which electrical charges are provided to a liquid or effluent and are used to generate a very fine aerosol of the effluent. Conventional electrospray techniques used in analytical applications involve different apparatus. In one conventional approach, a sharply pointed or tapered hollow tube (such as a syringe needle) can be used. A high voltage power source can be attached to the tube so that when the effluent is pumped through the tube, the effluent sprays out of the end in a very fine aerosol form. The droplets of the effluent aerosol then fly towards the counter electrode used as the collector for analysis. In some conventional alternatives, the voltage is introduced by passing the effluent through a spray tip tube which is coated with a conductive material (such as metal or graphite) at or near the exit orifice and then the coating of the tube is connected to a voltage source. One problem with this approach is the gradual ablation of the coating material through coronal discharge, such that the coating becomes less effective and the spray may become more erratic. A coated spray tip is also more expensive than one that is not coated with a conductive material.
One example of electrospray techniques and devices is described in U.S. Pat. No. 5,572,023, issued Nov. 5, 1996, to Caprioli, which is incorporated by reference herein. Caprioli describes the use of an electrically charged capillary spray needle. Before passing through the needle, the effluent in Caprioli passes through a steel fitting connected to a voltage source to add an electric charge to the fluid solution. The fluid then passes from the fitting through the non-conductive needle. In practice, this approach introduces additional dead volume into the system in the fitting and requires that the fitting used to introduce the charge to the fluid stream must be upstream of the needle.
Another example of apparatus and methods for electrospray applications is described in U.S. Pat. No. 4,842,701, which issued Jun. 27, 1989, to Smith et al., and which is hereby incorporated by reference herein. Smith describes the use of electrospray applications for chemical analysis of samples.
Nanospray applications involve the use of smaller sample sizes and volumes of solution to be analyzed. In such applications, the flow rate of the effluent is typically on the order of five (5) nanoliters per minute or so or even less. Such nanospray applications require that voltage be introduced into an effluent stream, which is complicated by the fact that microfluidic applications used a closed system for fluid delivery that is of the smallest practical internal volume. The point at which the voltage source is applied to the effluent stream is commonly referred to as the “liquid junction.” In addition, the reduced size of such samples is typically one or more orders of magnitude from more traditional electrospray applications, thus making it more difficult to provide accurate and precise apparatus which is not overly delicate and subject to breaking.
One conventional method for applying voltage to the effluent stream is to include a “Tee” type component or junction whereby the third port secures and suspends a conductive wire. This additional component increases the internal volume of the system, thereby degrading the performance of the analysis. Furthermore, the suspended wire in such conventional applications often introduces turbulence into the fluid flow, and thereby interferes with the direct, laminar flow of the effluent, causing undesirable delay.
Maintenance of such a conventional liquid junction often includes the replacement of the entire component or junction when it becomes clogged or when the conductive wire degrades to the point of inefficacy. This replacement usually involves disassembly of fittings and tubing, and thus can require a significant amount of labor and time.
An example of the use of a slightly different junction is found in U.S. Pat. No. 5,587,582, issued Dec. 24, 1996, to Henion et al., which is hereby incorporated by reference herein. Henion describes the use of a “T” shaped junction to introduce the charge to the fluid stream. In Henion, a T-shaped junction is provided by which a fluid from a capillary electrophoresis system is introduced to a capillary tube with a needle at the other end. At the junction of these two capillary tubes, a third tube introduces a solvent to the junction. The electrical charge is provided by attaching an electrode to the distal end of the tube with the needle. The assembly of Henion requires precise alignment and configuration of the tubes in order to minimize dead volume, as well as potential introducing turbulence into the fluid flow.
Still another approach is the use of a fractured tube for creating the spray. In U.S. Pat. No. 6,140,640, issued on Oct. 31, 2000 to Wittmer et al., the use of such a tube is described. In Wittmer, which is hereby incorporated by reference herein, a fracture is provided in the non-conductive tube, with the fracture located at a selected location from the needle or exit orifice of the tube, and a collar surrounds the tube near the fracture. The voltage is introduced to the fluid by means of a wire or electrode located near the fracture.
Such conventional approaches typically involve a number of difficulties. Fracturing a tube obviously weakens it and locating a fracture close enough to the needle and the wire to introduce a charge to the stream can require significant effort and delays. Conventional approaches using a T-shaped junction often involve undesirable dead volumes in the system, and can still require significant amounts of time for maintenance and replacement. Still other difficulties arise from the locations of the wires or electrodes used to apply the voltage to the stream in such conventional applications. It is undesirable to have exposed wires which can pose safety hazards or allow arcing.